At least one such cleaning machine is known and available on the market. It serves the purpose of opening the fibre flocks in the delivery air and removing the impurities therein. The fibre material is pulled over the bar grates and also beaten to some degree through the impact on the transfer chamber limiting walls, whereby impurities are separated from the material. Coarser impurities, as, for example, portions of shell, pass through the bar grates and are then sucked up. In the known machine, however, fine, dust-like impurities remain, at least in the delivery air current and leave the machine together with the fibre material conveyed with the delivery air.